How, then, can it be fairly said Buchanan took a “hardline” against secession?
It would make more sense to argue Buchanan took a faux hardline.
As a matter of constitutional law, Buchanan took a hard line against secession. In terms of using his presidential powers against it, Buchanan did nothing. That doesn't mean that he thought secession was constitutional or that he thought the cotton slave states were right to secede.
It's a subtle distinction between theory and practice, but most people would probably be able to grasp it. Maybe the word "hardliner," implying somebody who would do anything to defeat some measure or action, makes it hard to understand what's involved here. Think of it this way if you have to: Buchanan drew a firm line or a clear line or a solid, unequivocal line against secession in theory, in terms of constitutional law - without being any kind of "hardliner" in actual practice.
If you want to play Latter Day Confederate, though, don't run down Buchanan, he was the best thing the secessionists were going to get. And for all his faults and flaws and his utter failure as president, he did give them a little good advice about what to do and what not to do and why.